1. Field of the Invention
The present invention primarily relates to magnetic sensors each performing magnetization control of a free magnetic layer by an exchange bias method, and more particularly, relates to a magnetic sensor and a manufacturing method thereof, the magnetic sensor capable of improving reproduction sensitivity even when a track width is decreased and of suppressing the generation of side reading.
2. Description of the Related Art
FIG. 20 is a partial cross-sectional view showing the structure of a conventional magnetic sensor, viewed from a face opposing a recording medium.
Reference numeral 1 indicates a first antiferromagnetic layer, and on the antiferromagnetic layer 1, a fixed magnetic layer 2, a non-magnetic material layer 3, and a free magnetic layer 4 are formed in that order, thereby forming a multilayer film 7.
In the magnetic sensor shown in FIG. 20, on two side portions 4a of the free magnetic layer 4, second antiferromagnetic layers 5 are formed, and in addition, on the second antiferromagnetic layers 5, electrode layers 6 are formed.
As shown in FIG. 20, a track width Tw is determined by the distance between the bottom surfaces of the second antiferromagnetic layers 5 in the track width direction (X direction in the figure).
FIG. 21 is a partial plan of the magnetic sensor shown in FIG. 20 viewed from directly overhead. In this plan view, the electrode layers 6 shown in FIG. 20 are omitted. As shown in FIG. 21, the second antiferromagnetic layers 5 are formed on the entire two side portions 4a of the free magnetic layer 4.
FIG. 22 is a partial, vertical cross-sectional view of the magnetic sensor shown in FIG. 20, which is taken along the chain line shown in FIG. 20 in the direction parallel to the Y-Z plane and which is viewed along the direction indicated by the arrow E.
As shown in FIG. 22, a back end surface 7a of the multilayer film 7 is an inclined surface, and in a back side region in a height direction (Y direction in the figure) further from the back end surface 7a in the height direction, an insulating layer 8 is formed. In addition, it is understood that the second antiferromagnetic layer 5 is formed over the multilayer film 7 and the insulating layer 8.
In the magnetic sensor shown in FIG. 20, when an exchange coupling magnetic field is generated between the second antiferromagnetic layer 5 and each side portion 4a of the free magnetic layer 4, the side portions 4a of the free magnetic layer 4 are magnetically fixed in the X direction in the figure. As shown in FIG. 21, the second antiferromagnetic layers 5 covers the entire side portions 4a of the free magnetic layer 4, and the magnetizations of the entire side portions 4a are tightly fixed in the X direction shown in the figure by the exchange coupling magnetic fields described above.
In addition, a central portion 4b of the free magnetic layer 4 is placed in a single domain state in the X direction in the figure, which may be influenced by an external magnetic field, and when an external magnetic field penetrates into the central portion 4b described above from the Y direction in the figure, the magnetization of the central portion 4b of the free magnetic layer 4 rotates, so that the external magnetic field is detected by the magnetoresistive effect.
However, when the track width Tw is decreased in order to meet the trend toward a higher recording density, in the magnetic sensor shown in FIG. 20, the following problem has occurred.
In a method in which the magnetization control of the free magnetic layer 4 shown in FIG. 20 is performed by an exchange bias method using the second antiferromagnetic layers 5, the magnetization of the central portion 4b of the free magnetic layer 4 is aligned in the X direction shown in the figure by a bias magnetic field generated by an exchange interaction acting between adjacent spin electrons inside the free magnetic layer 4.
However, as the track width is decreased, a region strongly influenced by the bias magnetic field described above is relatively increased in the central portion 4b, and as a result, the reproduction sensitivity of the free magnetic layer 4 to an external magnetic field is disadvantageously decreased. In particular, since the vicinity of each of two ends 4b1 of the central portion 4b of the free magnetic layer 4 is strongly influenced by the bias magnetic field described above, magnetization inversion is not properly performed with respect to the external magnetic field, and as a result, the reproduction sensitivity is decreased as the track width is decreased.
As a method for solving the problem described above, a method in which the free magnetic layer 4 is formed from a material having a small exchange stiffness constant may be considered.
The exchange stiffness constant is a constant defining the magnitude of exchange energy with respect to spin distribution, and when a magnetic material has a smaller exchange stiffness constant, it becomes easier to rapidly change the directions of adjacent spin electrons. The “exchange stiffness constant” has been described in detail by S. Chikazui in “Kyojisei no Butsuri (II) (Physics of Ferromagnetic Material (II))”, pp. 166 to 169, (1984), published by Shokabo.
In order to improve the reproduction sensitivity, the exchange stiffness constant of the free magnetic layer 4 may be decreased as described above; however, when the exchange stiffness constant is excessively decreased, the Curie temperature is decreased, and as a result, thermal fluctuation of magnetization of the free magnetic layer 4 becomes a problem. As a result, the generation of Barkhausen noise or noise caused by ferromagnetic resonance induced by thermal fluctuation becomes serious.
In addition, when the exchange coupling magnetic field generated between the second antiferromagnetic layer 5 and the side portion 4a of the free magnetic layer 4 is decreased, the reproduction sensitivity can be improved. For example, when the film thickness of the second antiferromagnetic layer 5 is decreased, the exchange coupling magnetic field described above can be decreased.
However, when the exchange coupling magnetic field described above is decreased, the fixation of the magnetization of the side portion 4a is also decreased, and as a result, a problem of side reading may arise, which is generated by magnetization inversion in the side portion 4a caused by an external magnetic field.
Furthermore, there may also be mentioned a method for improving the reproduction sensitivity in which the film thickness of the free magnetic layer 4 is decreased so as to decrease the magnetic momentum (saturated magnetization Ms×film thickness t) per unit area of the free magnetic layer 4. However, when the film thickness of the free magnetic layer 4 is excessively decreased, a problem may arise in that the stability of reproduction waveforms is decreased or noise caused by thermal fluctuation is generated.